Sometimes m3u files on Android could be deleted by the system. This happens because Android scans sd-card, reads m3u playlists and fixes them by deleting the links to files which could not be found on your sd-card. Since you have URLs instead of files in your playlist, Android thinks that playlist is empty and deletes it. To avoid this just create some folder, place there empty ".nomedia" file and your m3u playlist. After that Android will exclude that folder from scanning and won't touch the playlist.

Alternatively, you can place your playlist to some web server (Dropbox public link is one of the options) and specify its URL in the app.